1. Field of the Invention
This invention relates to automatic iris control of a camera and, more particularly, to such automatic control as a function of the actual image signal level rather than the average image brightness level.
2. Description of the Prior Art
Automatic iris control devices are known for cameras, such as video cameras. The purpose of such devices is to increase the aperture of the iris when the brightness level of the image picked up by the camera is relatively low and to close, or reduce the aperture when that brightness level is relatively high.
Typically, the camera in which the automatic iris control device is used also is provided with an automatic gain control (AGC) circuit. The AGC circuit amplifies the signal derived from the picked up image with a gain that is inversely related to the image signal level. Thus, the gain of the AGC circuit is increased if the amplified image signal level is too low and the gain is decreased if that amplified image signal level is too high. Since automatic iris control and automatic gain control are provided in the same camera, the control circuits therefor generally are designed such that the aperture control feature has priority over the gain control feature. That is, first the aperture is controlled as a function of the image signal level and then the AGC circuit is controlled as a function of the level of the signal derived from the image which has passed through the controlled aperture.
In a typical automatic iris control circuit, the average image signal level is compared to a reference and any difference therebetween is used to open or close the iris accordingly. More particularly, if the average image signal level exceeds the reference, the iris aperture is reduced and, conversely, if the average image signal level is less than the reference, the iris aperture is increased. In this way, the overall light level of the object being imaged is used to control the iris. It is appreciated that a reduction in the iris aperture tends to reduce the average level of the image signal and, contrarily, an increase in the iris aperture tends to increase the average image signal level. Since the AGC circuit operates on the image signal whose level is a function of the iris aperture, the signal-to-noise ratio is improved by reason of this aperture priority. Of course, the gain of the AGC circuit is increased when the image signal is derived from a low-lighted object, even when the iris aperture is opened to its maximum.
In the conventional iris control device, the iris is driven by a motor to which a signal derived from the difference between the average image signal level and a reference is supplied. This feedback loop is designed to control the iris aperture such that the average image signal level tends to become equal to the reference. However, if only a small portion of the object being imaged by the camera is extremely bright, the average image signal level derived therefrom may be relatively high even though the majority of the object being imaged exhibits low light. Because of this disproportionate contribution of the small bright area to the average image signal level, the iris aperture may be reduced more than is desired. Consequently, the image signal level likewise is reduced in magnitude and the AGC circuit, even if supplied with high gain because of this reduced image signal level, may not be sufficient to compensate for this reduction in brightness. Therefore, the overall video picture which may be produced by this camera will be too dark.
If, instead of relying upon the average image signal level to control the iris, the peak level of the image signal is used, the iris may be subjected to a rapidly fluctuating control signal. But, the motor normally used to control the iris generally is not adequately responsive to a rapidly varying signal. Hence, satisfactory iris control may not be achieved by relying upon peak levels of the image signal.